1. Field of Invention
The currently claimed embodiments of the invention relate to magnetic resonance imaging systems, and more particularly to magnetic resonance imaging systems that provide images of intracranial vessel walls.
2. Discussion of Related Art
The presence of intracranial vascular disease is highly predictive of stroke(1). However, disease prevalence may be underestimated due to the lack of an appropriate diagnostic tool to depict the intracranial vessel wall(2). Black blood MR imaging (BBMRI) has emerged as an effective method to measure wall thickness and identify pathological features of extracranial vessels(3-5). Recently, its application has been extended to evaluate intracranial vessels, specifically to detect atherosclerosis(6-9) and vasculitis(7,10). Measuring intracranial vessel wall thickness remains a technical challenge given the small size of these vessels. Furthermore, the techniques introduced thus far have been standard 2D black blood sequences, which are prone to partial volume artifacts amplified by the inherent curving course of intracranial vessels(11). This adds to the challenge of covering the numerous intracranial sites that are prone to atherosclerosis formation (e.g., basilar artery (BA), middle cerebral artery (MCA) and petrous internal carotid artery (ICA)(12)by 2D imaging.
Three-dimensional acquisitions enable high isotropic resolution that can minimize the overestimation of wall thickness as a consequence of the tortuosity of these small vessels; however, 3D techniques suffer from long scan times and suboptimal flow suppression(13). For example, double inversion recovery techniques(14,15) typically employed in 2D acquisitions generally provide inadequate flow suppression in 3D acquisitions because of the relatively thick re-inversion pulse required. Furthermore, the long echo train length (ETL) used to suppress flow by dephasing effects in 2D turbo spin echo (TSE) techniques(16) are not possible at 3D without impractically long scan times. A recently proposed 3D technique, Volumetric ISotropic TSE Acquisition (VISTA, Philips), employs variable-flip-angle refocusing pulses to achieve a longer ETL for more effective flow suppression without compromising signal and at relatively short scan times(17). In fact, this technique has been shown to have higher signal-to-noise ratio (SNR) efficiency and stronger black-blood effects compared with conventional 3D TSE sequences(17-19).
A 3D variable flip-angle refocusing pulse sequence has been employed to image carotid(19) and peripheral(20) arterial walls. However, one cannot intuit the successful application of this technique to intracranial wall imaging since these vessels are structurally unique. For example, they are surrounded by cerebrospinal fluid (CSF) rather than soft tissue (e.g. fat). Therefore, there remains a need for improved MRI systems for imaging intracranial vessel walls.